1. Field of the Invention
Our invention pertains to a method of erasing or deliberately removing information from the storage target of a scan converter storage tube. The storage target for use with the method of our invention is of the type having a substrate fabricated from a single crystal of electrically insulating material such as, typically, sapphire. The storage target of this character is described and claimed in Takefumi Kato et al. copending U.S. patent application Ser. No. 890,495, "Storage Target for Scan Converter Tubes," filed on Mar. 27, 1978 and assigned to the instant assignee.
2. Description of the Prior Art
Scan converter storage tubes of various constructional and operational characteristics have been suggested and used for the storage, and later extraction, of data such as pictures and high-speed electrical signals. The storage target in a typical one (shown in FIG. 1 of the accompanying drawings) of such prior art scan converter storage tubes includes a silicon dioxide (SiO.sub.2) storage layer in the form of stripes or lands on one surface of a silicon substrate, on the opposite surface of which is a conductive backplate or a backing electrode. Alternatively the storage target as heretofore constructed includes a layer of collector electrode, having a set of regularly patterned openings formed therein, overlying a glass substrate.
This type of scan converter storage tube permits the storage (writing) of information in the form of an electric charge pattern and the nondestructive extraction (reading) of the information. The storage tube effects writing by taking advantage of the emission of secondary electrons from the SiO.sub.2 storage layer or from the glass substrate. The secondary emission ratio of the SiO.sub.2 storage layer or of the glass substrate, therefore, limits the writing speed of the storage tube, up to the order of several megahertz in terms of frequency. For this reason the conventional storage tube with its storage target of the noted constructions has not permitted the writing of rapidly changing phenomena or irregular or intermittent high-frequency signals, restricting its use to the storage of pictures that can be written at comparatively low speed.
The electromagnetic deflection of the electron beam in the prior art scan converter storage tube in question has also set a limit on its writing speed. Even were it not for this additional limitation, however, the secondary emission ratio of the conventional storage target would by itself keep the writing speed as low as several megahertz.
With a view to eliminating the above drawback of the prior art, the aforementioned U.S. patent application Ser. No. 890,495 proposed the improved storage target whose substrate was made of a single crystal of insulating material. As has later proved, however, a scan converter storage tube incorporating this improved storage target does not allow effective erasure of the stored information by the conventional method. A brief explanation follows on this conventional erasing method.
The various parts of the electron gun included in the storage tube are first set at appropriate working potentials. For example, the cathode may be set at ground potential, the control grid at -75 through 0 volt (V), the acceleration electrode at 350 V, the collimation electrode at 300 V, and the field mesh electrode at 650 V.
As the first step of erasure, aimed at priming the storage target, the electron gun bombards or scans the complete target surface with an unmodulated electron beam. In this first step a voltage of, for example, 300 V is impressed to the collector electrode of the storage target so that the potential of the storage surface of the target substrate may exceed a "first crossover potential" at which the secondary emission ratio of the substrate first becomes unity.
The second step of erasure aims at establishment of a "prewrite" or "erase" potential difference between collector electrode and storage surface, preparatory to the subsequent writing process. In this second step the storage target is again scanned with the unmodulated beam from the electron gun, this time with the application to the collector electrode of a voltage (e.g., 10 V) such that the storage surface potential may become less than the first crossover potential. As has been mentioned, this conventional erasing method does not enable ready, complete erasure of information when applied to the improved storage target.
With the conventional erasing method applied to the conventional storage target, the storage surface potential does become higher than the first crossover potential upon application of 300 V to the collector electrode. When struck with the unmodulated priming beam, therefore, the storage surface assumes the same potential (i.e., 300 V) as the collector electrode. The complete storage surface thus acquires the uniform potential regardless of whether or not information has been stored thereon.
In the subsequent second step of the conventional erasing method the storage surface potential is made less than the first crossover potential of the substrate. Thus the electron bombardment of this storage surface makes its potential equal to the cathode potential (i.e., 0 V). The result is the establishment of the desired 10 V prewrite potential difference between storage surface and collector electrode, with the 10 V charge on the storage member of the target.
The following seems to us the most reasonable explanation for the fact that the above conventional method of erasure is not applicable to the improved storage target of the mentioned copending U.S. application with any favorable results. In the noted first step of the conventional method the potential of the field mesh electrode is set considerably higher than that of the collector electrode. Upon electron bombardment of the storage surface, therefore, the secondary electrons excited therefrom are captured not only by the collector electrode but also by the field mesh electrode. The result of this is the higher potential of the storage surface than that of the collector electrode. The storage surface retains this high potential because it is of a monocrystalline insulator and so is highly insulating.
Let Vp be the difference between storage surface potential Vs and collector electrode potential Vc after the scanning of the storage surface with the priming beam. Then Vs approximately equals the sum of Vc and Vp. The storage surface potential is higher as aforesaid than the collector electrode potential.
In the second step of the conventional method the collector electrode potential with respect to the cathode potential is to be set lower than the first crossover potential in order to make the storage surface potential less than the first crossover potential. For the above stated reason, however, the storge surface potential actually does not become less than the first crossover potential if the collector electrode potential is set as above. With the storage surface potential being thus held higher than the first crossover potential, the electron bombardment of the storage surface fails to make its potential equal to the cathode potential.